The present invention relates in general to the field of hologram production and display and, more particularly, to a system and method for diverting at least a portion of a reference beam from impinging upon a diffuser disposed adjacent to holographic recording material.
One-step hologram (including holographic stereogram) production technology has been used to satisfactorily record holograms without the traditional step of creating preliminary holograms. Both computer image holograms and non-computer image holograms may be produced by such one-step technology. In some one-step systems, computer processed images of objects or computer models of objects allow the respective system to build a hologram from a number of contiguous, small, elemental pieces known as elemental holograms or hogels. To record each hogel on holographic recording material, an object beam is conditioned through the rendered image and interfered with by a reference beam. Examples of techniques for one-step hologram production can be found in the U.S. Patent Application entitled xe2x80x9cMethod and Apparatus for Recording One-Step, Full-Color, Full-Parallax, Holographic Stereograms,xe2x80x9d Ser. No. 09/098,581, naming Michael A. Klug, Mark E. Holzbach, and Alejandro J. Ferdman as inventors, and filed on Jun. 17, 1998, which is hereby incorporated by reference herein in its entirety.
In many holographic recording systems, and particularly in one-step reflection holographic recording systems, a diffuser is used to evenly distribute light in the object beam on to the holographic recording material. Typically, the diffuser is an anisotropic diffuser. To achieve a high quality hologram, the diffuser is placed as close as is possible to the holographic recording material. In recording a reflection hologram, the reference beam is directed at the holographic recording material from the opposite side as the object beam. Because of the closeness of the diffuser to the holographic recording material, the reference beam passes through the holographic recording material and impinges upon the surface of the diffuser. The surface of the diffuser usually reflects light from the reference beam back through the holographic recording material a second time. Moreover, because of the nature of the diffuser, the reflected light from the reference beam is typically reflected at a variety of angles.
The reflected light from the reference beam can be reflected such that it interferes with the reference beam as it traverses the holographic recording material. This problem is illustrated in FIG. 1. Light from reference beam 25 passes through holographic recording material 70 and is reflected by diffuser 58 as reflected reference beam portions 125. That interference pattern is recorded in the holographic recording material, resulting in an undesirable artifact that resembles a vertical line seemingly positioned infinitely deep with respect to the hologram plane. This results from the recording of a single beam hologram of the diffuser surface. This artifact is both distracting to the viewer and damaging to the diffraction efficiency, and thus the brightness, of the image. Additionally, reflected light from the reference beam can be reflected such that it interferes with the object beam, potentially creating interference patterns that are recorded in the holographic recording material. While in principle, those recorded interference patterns are similar to the interference patterns that are intended to be recorded (i.e., the interference pattern created by the original, un-reflected, reference beam and the object beam), the fact that the interference patterns were formed using light reflected from the reference beam means that additional distortion or unwanted artifacts might be present.
A number of strategies have been used to reduce and/or eliminate the problem of interaction between the reference beam and the diffuser. One solution is to place an anti reflection coating on the diffuser surface. However, anti-reflective coatings usually are effective only for particular bandwidths of wavelengths and certain angles of incidence of incoming light. Due to the extreme and varied angles at which a reference beam may strike a diffuser, and due to the fact that some diffusers are volumetric devices that have no surface relief, this technique has not proven successful. In practice, anti-reflective coatings have proven to eliminate only about 30% of reflected reference beam light, whereas to eliminate the artifacts described above, a greater percentage of the reflected reference beam light should be eliminated. Furthermore, anti-reflective coatings are difficult to uniformly apply over large areas such as the surface area of a diffuser.
Another technique is the use of a light control or xe2x80x9clouver screenxe2x80x9d film between the diffuser and the associated holographic recording material. As illustrated in FIG. 2, light from reference beam 25 passes through holographic recording material 70 and impinges upon louver screen film 59, where the light is absorbed, and/or generally prevented from reflecting back toward reference beam 25 by louvers 159. Object beam 20 passes through diffuser 58 and, because of the structure of louvers 159, generally passes through louver screen film 59. The result is diffused object beam 120. Louver screen film is a commercially available volumetric substrate that typically contains microscopic opaque strips or louvers, arranged in a parallel formation at a selected variable angle analogous to a venetian blind arrangement. Louver screen film is chosen with a particular blind spacing and angle that allows passage of the object beam light, for example, at angles of zero to plus or minus thirty degrees (xc2x130xc2x0), while absorbing reference beam light impinging at higher angles of, for example, approximately forty five degrees (45xc2x0). Such louver screen film successfully prevents reference beam light from impinging and reflecting off the surface of the diffuser, and thus eliminates the unwanted artifacts.
One problem associated with using louver screen film is the film""s requisite thickness (on the order of 1 mm) which necessarily further separates the diffuser from the surface of the holographic recording material. Because the louver screen film separates the diffuser and holographic recording material, the diffuser plane and the hologram plane are not as close together as is possible, which leads to poorer quality recorded holograms. Louver screen film may also introduce other artifacts into the hologram, due to the film""s periodicity and diffractive effects associated with the passage of light through the narrow louvers of the film. Finally, louver film often absorbs a significant percentage of the object beam light, again due to the existence of louvers within the film material, along with intrinsic substrate and surface absorption and reflection.
Accordingly, it is desirable to have a deflector that overcomes the deficiencies of the prior art, including for example, the thickness, efficiency, and ease of construction and use.
In accordance with teachings of the present invention, a system and method are provided to prevent at least a portion of a reference beam from intersecting with an associated diffuser. A holographic deflector incorporating teaching of the present invention may be placed between a diffuser and a holographic film or emulsion, similar to the placement of louver screen film. However, the holographic deflector preferably has a much thinner profile, requiring approximately 100-200 microns, as compared to the 1 mm needed to accommodate the louver film.
The holographic deflector is designed to deflect only light impinging on it from the particular angle that the reference beam strikes the holographic emulsion, and transmits nearly all other light strking it. These deflection characteristics are due to the Bragg selectivity of the holographic structure and its recording geometry.
A holographic deflector incorporating teaching of the present invention may have nearly 100% diffraction efficiency, ensuring the passage of little or no reference beam light to the diffuser. The holographic deflector allows the diffuser to be placed close to the holographic emulsion, enabling a closer approach to the ideal case of coincident vertical diffusion and hologram planes. Close placement of the diffuser to the emulsion also reduces another image artifact that is produced when rays from different but neighboring points in the object beam intersect prior to propagation through the holographic emulsion. Such a situation gives rise to a coarse fringe pattern, which manifests itself as dark horizontal lines in the image. Finally, the holographic deflector does not impart any visible artifacts of its own into the fine structure of the image as does louver film. This advantage is due to the microscopic nature of the fringes in a holographic optical element, as compared to the macroscopic structure of the louver film.
Accordingly, one aspect of the present invention provides a system for recording a hologram in a holographic recording material. The holographic recording material has at least a portion including a first surface and a second surface. The system includes a diffuser and a holographic deflector. The diffuser is disposed adjacent to the second surface whereby an object beam directed at the second surface can pass through the diffuser prior to contacting the holographic recording material. The holographic deflector is disposed between the second surface and the diffuser to prevent at least a portion of a reference beam directed at the first surface from impinging on the diffuser.
In another aspect of the present invention, a method for recording a hologram in holographic recording material is disclosed. The holographic recording material has at least a portion disposed in a plane having a first surface and a second surface. A reference beam is directed at the first surface. An object beam is directed at the second surface. A diffuser is disposed adjacent to the second surface whereby the object beam will pass through the diffuser prior to contacting the holographic recording material. At least a portion of a reference beam is deflected with a holographic deflector disposed between the second surface and the diffuser to prevent the reference beam from impinging on the diffuser. An interference pattern formed by the reference beam and the object beam is recorded in the holographic recording material.